Electrochemical energy storage devices include electrochemical capacitors and lithium batteries. An electric double-layer capacitor (EDLC), also known as a “supercapacitor” or “ultracapacitor”, is a type of electrochemical capacitor, which is characterized by a very high energy density relative to conventional capacitors. Instead of two metal plates separated by a regular dielectric material, an EDLC involves the separation of charges in a double electric field formed at the interface between an electrolyte and a high surface area conductor. A basic EDLC cell configuration is a pair of highly porous electrodes, typically including activated carbon, disposed on opposite faces of parallel conductive plates known as current collectors. The electrodes are impregnated with an electrolyte, and separated by a separator consisting of a porous electrically-insulating and ion-permeable membrane. When a voltage is applied between the electrodes, negative ions from the electrolyte flow to the positive electrode while positive ions from the electrolyte flow to the negative electrode, such that an electric double layer is formed at each electrode/electrolyte interface by the accumulated ionic charges. As a result, energy is stored by the separation of positive and negative charges at each interface. The separator prevents electrical contact between the conductive electrodes but allows the exchange of ions. When the EDLC is discharged, such as by powering an external electrical device, the voltage across the electrodes results in current flow as the ions discharge from the electrode surfaces. The EDLC may be recharged and discharged again over multiple charge cycles.
The extremely high surface area of the activated carbon electrodes, combined with a separation distance between electric double layers on the order of nanometers (compared with millimeters for electrostatic capacitors and micrometers for electrolytic capacitors), enables the absorption of a large number of ions per unit mass and, thus, an energy density that is orders of magnitude greater than that of conventional capacitors. The electrolyte may be an aqueous-based solution (e.g., a water solution of potassium hydroxide (KOH) or sulfuric acid (H2SO4)) or organic-based (e.g., acetonitrile (CH3CN), polypropylene carbonate). In an aqueous-based electrolyte, the voltage is limited to approximately 1V (above which water decomposes), whereas organic-based electrolytes have a higher maximum voltage of about 2.5-3.0V. Since each individual EDLC cell is limited to a relatively low voltage, multiple EDLC cells may be connected in series to enable higher voltage operation. However, serial connection reduces the total capacitance and also requires voltage-balancing.
While the amount of energy stored per unit weight is generally lower in an EDLC in comparison to electrochemical batteries, the EDLC has a much greater power density and a high charge/discharge rate. Furthermore, an EDLC has a far longer lifespan than a battery and can undergo many more charge cycles with little degradation (millions of charge cycles, compared to hundreds for common rechargeable batteries). Consequently, EDLCs are ideal for applications that require frequent and rapid power delivery, such as hybrid vehicles that are constantly braking and accelerating, while batteries are used to supply a larger amount of energy over a longer period of time. EDLCs are also environmentally friendly (have a long lifespan and are recyclable), safe (no corrosive electrolytes and other toxic materials requiring safe disposal), lightweight, and have a very low internal resistance (ESR). The charging process of an EDLC is also relative simple, as it draws only is the required amount and is not subject to overcharging. An EDLC has a higher self-discharge compared to other capacitors and electrochemical batteries.
Advances in materials and manufacturing methods in recent years have led to improved performance and lower cost of EDLCs, and to their utilization in various applications. For example, EDLCs can be employed to operate low-power electrical equipment, and to provide peak-load enhancement for hybrid or fuel-cell vehicles. EDLCs are also commonly used to complement batteries, such as in order to bridge short power interruptions in an uninterruptible power supply.
Graphene is a carbon allotrope, structured as a planar sheet of sp2-bonded carbon atoms densely packed in a honeycomb crystal lattice. Due to the unique intrinsic properties of graphene, there has been significant interest and research worldwide into the potential utilization of graphene in various nanomaterial applications, particularly with regard to the development of high-performance devices for energy storage and conversion. Graphene exhibits an extremely high surface area to mass ratio, as well as exceptional mechanical strength and flexibility (i.e., having a breaking strength over 100 times greater than a steel film of corresponding thickness). Furthermore, graphene possesses very high electrical conductivity and carrier mobility, and high optical transparency. Consequently, graphene has found use in a variety of electronic components, such as integrated circuits, solar cells, and display screens, as well as electrodes for ultracapacitors and Li-ion batteries. Due to its two-dimensional nature, the properties of graphene are highly anisotropic between the in-plane and out-of-plane directions (e.g., the conductivity perpendicular to the surface is significantly different than the conductivity along the surface). To overcome this effect, three-dimensional graphene architectures have been developed that incorporate carbon nanotube (CNT)-pillared graphene network structures, or interconnected CNT truss-like structures with networked graphene layers. For example, Yu, D.; Dai, L., “Self-assembled graphene/carbon nanotube hybrid films for supercapacitors” J. Phys. Chem. Letters 1, 2010, 467-470, discloses a solution layer-by-layer self-assembly approach to prepare multilayered hybrid carbon films of poly(ethyleneimine)-modified graphene sheets and acid-oxidized multi-walled CNTs to construct supercapacitors with an average specific capacitance of 120,000 F/kg.
Fan, Z. et al., “A Three-Dimensional Carbon Nanotube/Graphene Sandwich and its Application as Electrode in Supercapacitor”, Adv. Mater. 2010, 22, 3723-3728, also discloses the preparation of 3D CNT/graphene sandwich (CGS) structures with CNT pillars grown in between the graphene layers by chemical vapour deposition (CVD). A supercapacitor electrode based on such CGS exhibits a specific capacitance of 385 F/g at a scan rate of 10 mV/s in 6 M KOH aqueous solution.
Du, F. et al., “Preparation of Tunable 3D Pillared Carbon Nanotube-Graphene Networks for High-Performance Capacitance”, Chem. Mater. 2011, 23, 4810-4816, discloses the development of 3D pillared vertically aligned carbon nanotube (VACNT)-graphene architectures, by growing VACNTs between graphitic layers in thermally expanded highly ordered pyrolytic graphite (HOPG). By controlling the fabrication process, the length of the VACNT pillars can be tuned. The pillar length (PL) can be further tuned through hybridization with other functional nanomaterials, such as nickel hydroxide, by a simple electrodeposition process.
U.S. Pat. No. 6,031,711 to Tennent et al, entitled “Graphitic nanofibers in electrochemical capacitors”, is directed to an electrochemical capacitor with electrodes comprising carbon nanofibers having a high surface area (e.g., greater than 100 m2/gm) and being substantially free of micropores. The nanofibers may be functionalized with at least one functional group of: quinine, hydroquinone, quaternized aromatic amines, mercaptans, or disulfides. The functional groups may be contained in a ladder formula, which may include a graphenic analogue of quinine, napthaline disulfide, or dimethyl pyrazine. The carbon nanofibers may be substantially cylindrical with a substantially constant diameter, having graphitic layers concentric with the nanofiber axis and being substantially free of pyrolytically deposited carbon. The nanofibers may be coated with a thin coating layer of a pyrolyzed carbonaceous polymer.
European Patent No. 786,786 to Varakin, entitled “Capacitor with a double electrical layer”, discloses an EDLC with one electrode made of nickel oxide and the other electrode made of a fibrous carbonic material, preferably nickel-plated or copper-plated. The electrolyte is an aqueous solution of an alkali metal carbonate or hydroxide.
U.S. Patent Application Publication No. 2009/0176079 to Cabrera-Perez et al, entitled “Process for the production of porous carbon moldings”, discloses a process based on phase separation for producing porous carbon moldings. The process includes: preparing a mixture comprising at least one carbon former and one organic polymer in an organic solvent; evaporating the solvent until a viscous or highly viscous material or a corresponding molding is obtained; optionally shaping the material or moulding; and heating the material or moulding to temperatures between 200° C. and 4000° C. Following carbonization or pyrolysis, the carbon former and organic polymer may be converted into non-graphitic carbon or graphite.
U.S. Patent Application Publication No. 2010/0021819 to Zhamu et al, entitled “Graphene nanocomposites for electrochemical cell electrodes”, is directed to a graphene nanocomposite material for use in an electrochemical cell electrode, such as a supercapacitor electrode. The composition includes a solid particle of nano-scaled graphene platelets (NGPs) dispersed in, or bonded by, a first matrix or binder material. The NGPs occupy a weight fraction of 2% to 98% of the total nanocomposite weight, and are not obtained by graphitizing the binder or matrix material. Multiple solid particles are bonded by a second binder material. The binder materials may include: a polymer, polymeric carbon, amorphous carbon, metal, glass, ceramic, oxide, and/or organic material. The solid particles may include microscopic or meso-scale pores to accommodate electrolyte.
U.S. Patent Application Publication No. 2011/0183180 to Yu et al, entitled “Flexible asymmetric electrochemical cells using nano graphene platelet as an electrode material”, is directed to nano graphene platelet (NGP) based electrodes for supercapacitors or supercapacitor-battery hybrid electrochemical cells. The cell includes: a sheet of graphene paper as a first electrode including NGPs having a platelet thickness less than 1 nm, the first electrode having electrolyte-accessible pores; a thin-film or paper-like first separator and electrolyte; and a thin-film or paper-like second electrode which is different in composition from the first electrode. The separator is sandwiched between the first and second electrode to form a flexible laminate configuration. The electrodes may include a binder material that bonds graphene platelets together to form a cohered nanocomposite layer.
U.S. Patent Application Publication No. 2011/0121264 to Choi et al, entitled “Composite structure of graphene and nanostructure and method of manufacturing the same”, is directed to composite structures of graphene disposed with at least one one-dimensional nanostructure, such as nanowires, nanotubes, and/or nanorods. In one embodiment, the nanostructure is disposed substantially perpendicular to and inclined with respect to a first graphene and a second graphene spaced apart from the first graphene, and an insulating material fills in the spaces left by the nanostructure.
U.S. Patent Application Publication No. 2013/0295374 to Tang et al, entitled “Graphene sheet film connected with carbon nanotubes, method for producing same, and graphene sheet capacitor using same”, discloses a graphene sheet assembly film with multiple graphene sheet laminates, each of which includes two or more graphene sheets laminated parallel to each other via first carbon nanotubes. The graphene sheet laminates are electrically and mechanically three-dimensionally connected to each other via second carbon nanotubes.
Han T. H., et al., “Peptide/Graphene Hybrid Assembly into Core/Shell Nanowires”, Advanced Materials, Vol. 22, 2010, pp. 2060-2064, discloses an approach for producing peptide/graphene core/shell nanowires by single-step solution processing. The aromatic peptide of diphenylalanine, which was found to self-assemble into highly stable nanoscale morphologies such as nanotubes, nanowires, and nanoribbons, is used. The resultant hybrid nanowires were electroconductive, and were further processed to create a hollow graphene-shell network that could be employed as a supercapacitor electrode.